Circular Dichroism of DNA.

The circular dichroism of sixteen DNA samples from fourteen species has been determined. Plots of molar ellipticity against base composition show considerable scatter, which is much too great to be accounted for in terms of reproducibility of materials and measurements. The data are most simply fitted with the assumption of linearity; extrapolation then leads to apparent, circular dichroism curves for 0 and 100% (A+T). These are regarded as having probably no physical reality, but serve as standards for the calculation of circular dichroism curves for DNA of any given composition. For the materials used in the present work the discrepancy between the observed curves and those calculated from the formal standards is at worst not more than about 10%, and much better than this over most of the range, particularly on the long-wavelength side of the crossover. The use of a quadratic function, based on the assumption that the circular dichroism can be operationally expressed, like hypochromicity, in terms of three spectroscopic parameters, offers no significant advantage in describing the circular dichroism data. [ABSTRACT FROM AUTHOR]